{
  "bibcode": "2024GCN.36631....1O",
  "eventId": "GRB 240606B",
  "subject": "GRB 240606B: Swift-XRT afterglow detection",
  "body": "J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), M.G. Bernardini\n(INAF-OAB), E. Ambrosi\t(INAF-IASFPA) , M. Capalbi (INAF-IASFPA), D.N.\nBurrows (PSU), J. D. Gropp (PSU), S. Dichiara (PSU), A.P. Beardmore (U.\nLeicester) and P.A. Evans (U. Leicester) reports on behalf of the\nSwift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nSwift/BAT-GUANO-detected burst GRB 240606B, collecting 4.0 ks of Photon\nCounting (PC) mode data between T0+22.9 ks and T0+35.1 ks. \n\nFour uncatalogued X-ray sources are detected consistent with being\nwithin 493 arcsec of the BAT-GUANO position, of which one (\"Source 1\")\nis fading with 2.1 sigma significance and is undetected by eRosita to\ndeeper upper limits. It is likely the afterglow.\n\nUsing 3507 s of PC mode data and 3 UVOT images, we find an enhanced XRT\nposition (using the XRT-UVOT alignment and matching UVOT field sources\nto the USNO-B1 catalogue): RA, Dec = 40.29507, -60.61201 which is\nequivalent to:\n\nRA (J2000): 02h 41m 10.82s\nDec(J2000): -60d 36' 43.2\"\n\nwith an uncertainty of 2.4 arcsec (radius, 90% confidence). This\nposition is 53 arcsec from the BAT-GUANO position. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=1.2 (+0.9, -1.0).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.7 (+0.4, -0.3). The\nbest-fitting absorption column is  6 (+11, -4) x 10^20 cm^-2,\nconsistent with the Galactic value of 2.4 x 10^20 cm^-2 (Willingale et\nal. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux\nconversion factor deduced from this spectrum  is 4.1 x 10^-11 (4.5 x\n10^-11) erg cm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     6 (+11, -4) x 10^20 cm^-2\nGalactic foreground: 2.4 x 10^20 cm^-2\nExcess significance: <1.6 sigma\nPhoton index:\t     1.7 (+0.4, -0.3)\n\nIf the light curve continues to decay with a power-law decay index of\n1.2, the count rate at T+24 hours will be 0.013 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.2 x\n10^-13 (5.7 x 10^-13) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00021695.\nThe results of the full analysis of the XRT observations are available\nat https://www.swift.ac.uk/ToO_GRBs/00021695.\n\nThis circular is an official product of the Swift-XRT team.\n\n",
  "submitter": "Phil Evans at U of Leicester <pae9@star.le.ac.uk>",
  "submittedHow": "email",
  "createdOn": 1717729967396,
  "circularId": 36631
}